This application claims priority to an application entitled xe2x80x9cInitial Acquisition and Frame Synchronization in Spread Spectrum Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Jul. 21, 1998 and assigned Ser. No. 98-29344, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a spread spectrum communication system, and, in particular, to a device and method for performing initial acquisition and frame synchronization using a spreading code for a mobile station.
2. Description of the Related Art
FIG. 1 illustrates an IS-95 forward link of a base station, for transmitting channel signals to a mobile station in a Code Division Multiple Access (CDMA) mobile communication system. As shown, in a CDMA mobile communication system, the forward link includes a pilot channel, a sync channel and a paging channel. Though not illustrated, the forward link further includes a traffic channel for transmitting the voice and data of a user.
Referring to FIG. 1, a pilot channel generator 110 generates a pilot signal comprised of all xe2x80x9c1xe2x80x9ds for a pilot channel, and a multiplier 114 multiplies the pilot signal by an orthogonal code W0 to orthogonally spread the pilot signal. Here, a specific Walsh code is used for the orthogonal code W0. A multiplier 115 multiplies the pilot channel signal output from the multiplier 114 by a PN (Pseudo Noise) sequence to spread the pilot channel signal.
With regard to the structure of a sync channel generator 120, a coding rate R=xc2xd, constraint length K=9 convolutional encoder can be used for an encoder 121. A repeater 122 repeats sync symbols output from the encoder 121 N times (N=2). An interleaver 123 interleaves the symbols output from the repeater 122 in order to prevent burst errors. A block interleaver is typically used for the interleaver 123. A multiplier 124 multiplies the sync channel signal by a specific orthogonal code assigned to the sync channel to orthogonally spread the sync channel signal. The sync channel outputs the positional information, standard time information and long code information of the base station, and also outputs information for system synchronization between the base station and a mobile station. As stated above, the sync channel generator 120 encodes an input sync channel signal, and multiplies the encoded sync channel signal by a specific Walsh code Wsync assigned to the sync channel out of available Walsh codes to orthogonally spread the sync channel signal. A multiplier 125 multiplies the sync channel signal output from the multiplier 124 by the PN sequence to spread the sync channel signal.
With regard to a paging channel generator 130, an encoder 131 encodes an input paging channel signal. An R=xc2xd, K=9 convolutional encoder can be used for the encoder 131. A repeater 132 repeats the symbols output from the encoder 131 N times (N=1 or 2). An interleaver 133 interleaves the symbols output from the repeater 132 in order to prevent burst errors. A block interleaver is typically used for the interleaver 133. A long code generator 141 generates a long code which is the user identification code. A decimator 142 decimates the long code so as to match the rate of the long code to the rate of the symbol output from the interleaver 133. An exclusive OR gate 143 XORs the encoded paging signal output from the interleaver 133 and the long code output from the decimator 142 to scramble the paging signal. A multiplier 134 multiplies the scrambled paging signal output from the exclusive OR gate 143 by an orthogonal code Wp assigned to the paging channel in order to maintain orthogonality with other channel signal. A multiplier 135 multiplies the paging channel signal output from the multiplier 134 by the PN sequence to spread the paging channel signal.
As stated above, the orthogonally spread transmission signals of the respective channels are multiplied by the PN sequence to be spread, and up-converted into an RF (Radio Frequency) signal to be transmitted. In the IS-95 standard, spreading is performed using two different PN sequences for the I and Q arms. The PN sequences used herein have a period of 32,768.
In the forward link structure of FIG. 1, the pilot channel does not carry data and spreads a signal of all xe2x80x9c1xe2x80x9ds with a PN sequence of period 32,768 to transmit. In a system having a chip rate of 1.2288 Mcps (chips per second), one PN sequence period corresponds to 26.7 msec (80/3 msec). Upon power-on, the receiver in a mobile station acquires the pilot channel signal on the forward link shown in FIG. 1 in order to acquire synchronization with a base station.
FIG. 2 illustrates a receiver in a mobile station, which receives forward link channel signals from a base station.
Referring to FIG. 2, an RF receiver 212 receives an RF signal transmitted from a base station and then down-converts the received RF signal into a baseband signal. An analog-to-digital (A/D) converter 214 converts the baseband signal output from the RF receiver 212 to digital data. A searcher 222 acquires the pilot channel signal out of the forward channel signals in order to synchronize the mobile station with the base station. N fingers 231-23N despread corresponding forward channel signals to detect a correlation value among the channel signals. A combiner 226 combines the output signals of the respective fingers 231-23N.
As illustrated in FIG. 2, a receiver of a mobile station is comprised of the searcher 222, the N fingers 231-23N and the combiner 226. Acquisition of the pilot signal is performed by the searcher 222.
FIG. 3 is a timing diagram of forward channel signals that a base station transmits, in which the frame offset of a traffic channel is assumed to be 0.
Referring to FIG. 3, reference numeral 311 represents a 80 ms boundary of a base station, which is determined from a two second boundary of the Global Positioning System (GPS). Reference numeral 313 represents the pilot offset of the base station. Reference numeral 315 represents the boundaries of three spreading sequence periods within 80 ms, from which it is clear that one spreading sequence period is 26.7 ms (=80/3 ms). Herein, the spreading sequence is assumed to be a PN sequence. Each spreading sequence period is synchronized with a 26.7 ms frame boundary where a sync channel is interleaved. Here, the 80 ms frame will be referred to as the second frame and the 26.7 ms frame the first frame.
Reference numeral 317 represents an 80 ms frame boundary of the sync channel, and the 80 ms frame structure of the sync channel is illustrated in FIG. 4. For the sync channel signal, the 80 ms frame represented by reference numeral 412 is comprised of three 26.7 ms frames each including a sync bit SOM (Start of Message) set according to a pilot sequence period. For example, in the 80 ms period, the sync bit SOM for the first 26.7 ms frame period is determined as xe2x80x9c1xe2x80x9d (or xe2x80x9c0xe2x80x9d), and the sync bits SOMs for the following 26.7 ms frames are determined as xe2x80x9c0xe2x80x9d (or xe2x80x9c1xe2x80x9d). Therefore, detecting a sync bit SOM of xe2x80x9c1xe2x80x9d (or xe2x80x9c0xe2x80x9d) in the 80 ms period means detection of an 80 ms sync channel signal.
Reference numeral 319 represents the frame boundaries of the paging channel and the traffic channel. For the traffic channel, the 80 ms frame is comprised of four 20 ms frames. Therefore, it is noted from FIG. 3 that in the 80 ms period, the sync channel is comprised of three 26.7 ms frames and the traffic channel is comprised of four 20 ms frames.
Referring to FIGS. 3 and 4, a description will be made regarding the synchronizing procedure performed between a base station and a mobile station. The standard timing of the base station is derived from the 80 ms boundary 311 which is determined from the two second boundary of the GPS. The pilot signal of the base station is offset by the pilot offset 313 in the 80 ms boundary obtained from the GPS. This is to uniquely identify base stations using the same sequence by setting the pilot offset differently for each of the respective base stations. The pilot channel signals for the forward link are repeated at a period of 26.7 ms as represented by reference numeral 315. A sync channel signal is interleaved/deinterleaved at periods of 26.7 ms as represented by reference numeral 414, and this boundary is synchronized with one pilot sequence period. Therefore, upon acquiring a pilot channel signal, a mobile station in an IS-95 mobile communication system can accurately acquire the interleaving/deinterleaving frame sync for a sync channel as shown in FIG. 4.
Thereafter, the mobile station should acquire the 80 ms boundary 317 of the sync channel. The sync channel for the forward link transmits the sync bit SOM every 26.7 ms as represented by reference numeral 414. The SOM bit is set to xe2x80x9c1xe2x80x9d in the first 26.7 ms frame and to xe2x80x9c0xe2x80x9d in the following two 26.7 ms frames. The receiver of the mobile station becomes synchronized with the 80 ms boundary utilizing the SOM bits of the sync channel. The receiver of the mobile station synchronizes with the pilot channel in order to be synchronized with the base station, whereby the receiver demodulates a signal on the sync channel every 26.7 ms, and determines a 26.7 ms frame with the demodulated SOM bit of xe2x80x9c1xe2x80x9d as the start of an 80 ms boundary.
The forward link structure of FIG. 1 and the synchronization procedure of FIGS. 3 and 4 are applicable to an IS-95 mobile communication system having a chip rate of 1.2288 Mcps. However, for high-speed data transmission and effective system design, an IMT-2000 system will increase the chip rate to use the wider bandwidth.
It is expected that the IMT-2000 mobile communication system will use a chip rate which is higher by 3, 6 and 12 times the chip rate of the existing IS-95 system. Herein, it is assumed that the chip rate of the IMT-2000 system increases to 3.6864 Mcps, three times the chip rate of the IS-95 system. In this case, if a PN sequence having the same period as that of a spreading sequence for the existing IS-95 mobile communication system is used, one PN sequence period decreases by ⅓ times to be 80/9 ms. In that case, the procedure for acquiring the 80 ms sync for the sync channel becomes complicated. In particular, even though the mobile station initially acquires a pilot signal, since it does not know the boundary of the 26.7 ms frame, it is not possible to use the sync acquiring procedure used in the 1.2288 Mcps narrow band system.
One method for solving this problem is to use a spreading sequence having a period which is as long as the increase in the chip rate. For example, when the chip rate is increased by three times, the period of the spreading sequence is also increased by three times so as to maintain one spreading sequence period to be 26.7 ms. However, the increase in length of the PN sequence by three times causes an increase in initial acquisition time of the mobile station.
Therefore, when the chip rate increases beyond that of the existing IS-95 system, a new initial synchronization method will be required.
It is an object of the present invention to provide a device and method for rapidly performing initial acquisition and frame synchronization of a received signal in a spread spectrum communication system.
It is another object of the present invention to provide a device and method for rapidly performing initial acquisition and frame synchronization of a received signal using a spreading sequence having the same frame boundary during spreading in a receiver for a CDMA communication system.
In accordance with an object of the present invention, there is provided a device for transmitting a channel signal for a base station in a CDMA communication system. The signal includes a first chip rate that is multiple times a second chip rate, a first frame with a duration of a spreading sequence having the second chip rate, and a second frame whose frame length is multiple times the length of the first frame. The device enables the receiver to synchronize the spreading sequence having the first chip rate with the first frame. The device comprises a spreading sequence generator for generating a spreading sequence having the first chip rate; a sync pattern generator for generating a sync pattern for distinguishing the first frame by varying the pattern of the spreading sequence having the first chip rate at a boundary of the first frame; and a spreader for generating a sync spreading code using the spreading code having the first chip rate and the sync pattern, and spreading a transmission signal with the sync spreading code.
In addition, there is provided a device for receiving the channel signal in a mobile station in a CDMA communication system. The received signal includes a first chip rate that is multiple times the second chip rate, the first frame having the second chip rate and the second frame whose frame length is multiple times the length of the first frame. The device receives the spread signal using a spreading code having the first chip rate, which alternates its sign from one first frame duration to the next. The device comprises a despreader for despreading the spread signal with a spreading sequence having the first chip rate; an orthogonal demodulator for orthogonally demodulating a pilot channel signal from the despread signal; a decider for examining the pilot channel signal to determine whether the pilot channel signal has varied in sign, and, upon detection of variation in sign of the pilot channel signal, deciding a boundary of the first frame; and a sync channel receiver for determining a boundary of the first frame for the sync channel according to the output of the decider and detecting sync bits at predetermined positions in the first frame to acquire synchronization with the second frame.